Applications of Irreversible Thermodynamics to Transport Processes in Electrolyte Solutions

Non-equilibrium thermodynamics is applied to isothermal vector transport processes in concentrated aqueous binary electrolyte solutions, It is shown that this branch of thermodynamics gives rise to linear mobility coefficients, Lij, and frictional coefficients, Rij , which measure the effect on ion-constituent, i, of a thermodynamic force on ion-constituent, j. The cross coefficients, Lij and Rij where i = j , are of especial interest as they provide a measure of the interactions between species i and j. The transport coefficients are considered to he more fundamental than the more commonly reported transport properties which can he shown to be combinations of the Lij or the Rij. The interpretation of the concentration dependance of transport coefficients is qualitative and must therefore be based on a comparison between similar salts. Zinc chloride and zinc perchlorate were selected for experimental study as zinc chloride exhibits extensive self complexing in aqueous solution whereas zinc perchlorate does not. The effects of self-completing are therefore assessed using the experimental results for these two salts, in conjunction with transport coefficients available in the literature for several other salts. Four experimental measurements are required to evaluate the four independent transport coefficients of a binary electrolyte solution. These are of electrical conductivity, salt diffusion coefficient, Hittorf transference number, and cell emf transference number, as a function of concentration. Each of these experiments is analysed and equations are derived which express the mobility and frictional coefficients in terms of these experimental quantities. It is shown that the equality of the two transference numbers is a consequence of the Onsager reciprocal relations, which state that Lij = Lij and Rij = Rij for i = j . These relations require experimental proof which is given. The experimental section describes the preparation of aqueous solutions of zinc chloride and zinc perchlorate and the measurement of their transport properties. Electrical conductivity was measured using standard methods. Diffusion coefficients were measured using an optical technique which allowed the progress of restricted diffusion in a closed rectangular glass cell to be followed as a function of time. Eittorf transference numbers were measured for both salts using a glass cell based on the design of Maclnnes and Dole. Cell emf transference numbers were measured for zinc chloride only using cells of similar design to those described by Pikal and Miller. Values for zinc perchlorate were obtained from the literature. The experimental results are collected and used to calculate the transport coefficients for zinc chloride and zinc perchlorate. The relative merits of the mobility and frictional coefficients for interpretative purposes are discussed. Both schemes have merits, but the frictional representation has the advantage that it provides coefficients which measure the interactions between ions and solvent as well as those which measure interactions between ion and ion. Also the cation - anion frictional coefficient: reflects the difference between strong ion association, as exhibited by aqueous silver nitrate, and self-complexing, as exhibited by the zinc and cadmium halides, whereas the corresponding mobility coefficient does not. Finally it is demonstrated that the mobility coefficients of a self-complexing salt can be expressed as a combination of the mobility coefficients of each of the individual species. The concentration dependence of each of these latter coefficients can be estimated qualitatively from theoretical considerations and combined to explain experimental trends. In the final chapter the effect of the weak ion association in dilute aqueous solutions of 2:2 electrolytes on their diffusion coefficients is considered. It is shown that for the four salts for which experimental data are available the observed diffusion coefficient is equal to the diffusion coefficient calculated from electrolyte theory using the concentration of free ions in solution. This relation is found to be valid within experimental error to a total salt concentration of 0.1mol dm -1. Calculations were also carried for three weak organic acids for which experimental diffusion coefficients are available, but in these systems the above relation was found not to be valid